Tuesday, 21 May 2013

Know Thy Java Object Memory Layout

Sometimes we want to know how an object is laid out in memory, here's 2 use cases and 2 solutions which remove the need to hypothesise.
A few months back I was satisfying my OCD by reading up on java object memory layout. Now Java, as we all know and love, is all about taking care of such pesky details as memory layout for you. You just leave it to the JVM son, and don't lose sleep over it.
Sometimes though... sometimes you do care. And when you do, here's how to find out.

False False Sharing Protection

For one thing, the rules are not part of the JLS, they are just implementation details. If you read Martin Thompson's article about false sharing you'll notice Mr. T had a solution to false sharing which worked on JDK 6, but no longer worked on JDK 7. Here are the 2 versions:

It turns out the JVM changed the way it orders the fields between 6 and 7, and that was enough to break the spell. In fairness there is no rule specified above which requires the fields order to correlate to the order in which they were defined, but ... it's allot to worry about and it can trip you up.

I’ve tried to order the fields such that the risk of false-sharing is minimized. I am aware that Java 7 can re-order fields however. I’ve run the performance test using Martin Thompson’s PaddedAtomicLong instead but got no performance increase on Java 7. Perhaps I’ve missed something so feel free to try it yourself.

Now Nick is a savvy dude, and I'm not quoting him here to criticise him. I'm quoting him to show that this is confusing stuff (so in a way, I quote him to comfort myself in the company of others equally confused professionals). How can we know? here's one way I thought of after talking to Nick:

Using Unsafe I can get the field offset from the object reference, if 2 fields are less than a cache line apart they can suffer from false sharing (depending on the end location in memory). Sure, it's a bit of a hackish way to verify things, but it can become part of your build so in the case of version changes you on't get caught out.

Note that it's the runtime which will determine the layout, not the compiler, so if you build on version X and run on Y it won't help you much.

Enough of that false sharing thing... so negative... why would you care about memory layout apart from false sharing? Here's another example.

The Hot Bunch

Through the blessings of the gods, at about the same time LMAX released the CoalescingRingBuffer, Gil Tene (CTO of Azul) released HdrHistogram. Now Gil is seriously, awesomely, bright and knows more about JVMs than most mortals (here's his InfoQ talk, watch it) so I was keen to look into his code. And what do you know, a bunch of hot fields:

What Gil is doing here is good stuff, he's trying to get relevant fields to huddle together in memory, which will improve the likelihood of them ending up on the same cache line, saving the CPU a potential cache miss. Sadly the JVM has other plans...

So here is another tool to help make sense of your memory layout to add to your tool belt: Java Object Layout I bumped into it by accident, not while obsessing about memory layout at all. Here's the output for Histogram:

Note how histogramData jumps to the botton and subBucketMask is moved to the top, breaking up our hot bunch. The solution is ugly but effective, move all fields but the hot bunch to an otherwise pointless parent class:

And the new layout:

Joy! I shall be sending Mr. Tene a pull request shortly :-)

UPDATE 16/01/2014: The excellent JOL has now been released under OpenJDK here. It's even better than before and supports many a funky feature (worthy of a separate post). I've updated the links to point to the new project. Also check out Shipilev's blog post on heap dumps demonstrating the use of this tool.

I didn't and now had both you and Mr. Shipilev point it out and it is a great example. Not sure how you mean more interesting though, it's the same technique but padding both sides instead of one. I totally agree it's one the right ways to avoid false sharing.Code discussed is here: http://hg.openjdk.java.net/code-tools/jmh/file/tip/jmh-core/src/main/java/org/openjdk/jmh/logic/BlackHole.javaThanks :-)

I'd like to speak with you about the possibility of writing for No Starch Press (nostarch.com). We've published books like "Eloquent JavaScript," "Learn You a Haskell For Great Good!," "Land of Lisp," and so on. We have nothing current on Java.